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Polymorphism in Systematics and Comparative Biology P1: FIZ/FEA/FGI P2: Fne/FGO QC: FDS/anil T1: FDX September 17, 1999 15:27 Annual Reviews AR093-12 Annu. Rev. Ecol. Syst. 1999. 30:327–62 Copyright c 1999 by Annual Reviews. All rights reserved POLYMORPHISM IN SYSTEMATICS AND COMPARATIVE BIOLOGY John J. Wiens Section of Amphibians and Reptiles, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213-4080; e-mail: [email protected]. Key Words comparative methods, phylogenetic analysis, phylogeny, intraspecific variation, species-limits ■ Abstract Polymorphism, or variation within species, is common in all kinds of data and is the major focus of research on microevolution. However, polymorphism is often ignored by those who study macroevolution: systematists and comparative evolutionary biologists. Polymorphism may have a profound impact on phylogeny reconstruction, species-delimitation, and studies of character evolution. A variety of methods are used to deal with polymorphism in phylogeny reconstruction, and many of these methods have been extremely controversial for more than 20 years. Recent research has attempted to address the accuracy of these methods (their ability to es- timate the true phylogeny) and to resolve these issues, using computer simulation, congruence, and statistical analyses. These studies suggest three things: that (a) the exclusion of polymorphic characters (as is commonly done in morphological phylo- genetics) is unjustified and may greatly decrease accuracy relative to analyses that include these characters; (b) methods that incorporate frequency information on poly- morphic characters tend to perform best, and (c) distance and likelihood methods designed for polymorphic data may often outperform parsimony methods. Although rarely discussed, polymorphism may also have a major impact on comparative studies of character evolution, such as the reconstruction of ancestral character states. Finally, polymorphism is an important issue in the delimitation of species, although this area has been somewhat neglected methodologically. The integration of within-species variation and microevolutionary processes into studies of systematics and comparative evolutionary biology is another example of the benefits of exchange of ideas between the fields of population genetics and systematics. INTRODUCTION One of the most important trends in systematics and evolutionary biology in re- cent years has been an increasing appreciation for the interconnectedness of these fields. For example, phylogenies are used increasingly by evolutionary biolo- gists studying? ecology and behavior (e.g. 9, 60, 82), and systematists using DNA 0066-4162/99/1120-0327$08.00 327 P1: FIZ/FEA/FGI P2: Fne/FGO QC: FDS/anil T1: FDX September 17, 1999 15:27 Annual Reviews AR093-12 328 WIENS and RNA sequence data are beginning to incorporate more and more details of molecular evolutionary processes into their phylogenetic analyses (e.g. 130). One of the areas in which a phylogenetic approach has had an important impact is the study of within-species variation, particularly in the fields of phylogeography and molecular population genetics (e.g. 4, 45, 51, 62, 126). However, many un- resolved questions remain as to what the study of within-species variation and microevolutionary processes might have to offer between-species systematic and comparative evolutionary studies (e.g. 58). Heritable variation within species is the basic material of evolutionary change and the major subject of research on microevolutionary processes. Intraspecific variation is abundant in all kinds of phenotypic and genotypic traits, including morphology, behavior, allozymes, and DNA sequences. This variation is not really surprising because if characters vary between species, they must also vary within species, at least at some point in their evolution. In many cases, especially among closely related species, this instraspecific variation may persist and may be abundant. For example, among the nine species of the lizard genus Urosaurus,23 of 24 qualitative morphological characters that vary between species were found to vary within one or more species as well (136). I define polymorphism as variation within species that is (at least partly) inde- pendent of ontogeny and sex. I assume that this variation is genetically based and heritable, and for the purposes of this paper I deal primarily with variation in dis- crete or qualitative characters, rather than continuous variation in quantitative traits. Despite the prevalence of intraspecific variation, phylogenetic biologists have a long and continuing tradition of ignoring polymorphism. For example, mor- phological systematists often exclude characters that show any or “too much” variation within species (109a). Both molecular and morphological systematists often “avoid” or minimize polymorphism by sampling only a single individual per species. When polymorphism is dealt with explicitly, as in phylogenetic analyses of allozyme data and some studies of morphology, the appropriateness of different methods for phylogenetic analysis of these data is controversial and has been the subject of heated debate for over 20 years (e.g. 11, 12, 20, 33–35, 39, 40, 43, 75, 90–93, 96, 97, 116, 129, 130, 137–139, 142, 143). The controversy over the ef- ficacy of different methods for analyzing polymorphism is not merely academic because different methods may give very different estimates of phylogeny from the same data (Figure 1; 137). Different phylogenetic hypotheses may have very different implications for comparative evolutionary studies. But even if the tree is stable, different methods of treating within-species variation in ancestral state reconstructions may lead to radically different hypotheses about how traits evolve (see below). Descriptions of comparative methods designed for discrete traits (e.g. 76, 104, 118) rarely mention that these traits may vary within species or what the potential impact may be of this variation on the methods or results. Species-level systematics, or alpha taxonomy, also involves analyzing poly- morphic characters.? Analytically, the main task of species-level systematics is to P1: FIZ/FEA/FGI P2: Fne/FGO QC: FDS/anil T1: FDX September 17, 1999 15:27 Annual Reviews AR093-12 POLYMORPHISM IN SYSTEMATICS 329 Figure 1 Different methods for coding polymorphic characters for phylogenetic anal- ysis lead to very different hypotheses of evolutionary relationships. Results are based on morphological data for the lizard genus Urosaurus (136). Numbers at nodes indicate bootstrap values (42; bootstrap values <50% not shown). Each data set was analyzed with 1000 pseudoreplicates with the branch-and-bound search option. distinguish between intraspecific and interspecific character variation. The delim- itation, diagnosis, and description of species is at least as important an endeavor of systematics as phylogeny reconstruction. Yet, in contrast to phylogeny recon- struction, there has been relatively little methodological improvement in this area, especially as practiced by morphological systematists, who have described and will continue to describe most of the world’s species. Alpha taxonomy is a branch of systematics that would benefit tremendously from a more explicit treatment of polymorphism. In this paper, I review the implications of within-species variation for studies of systematics and comparative biology. I first provide an overview of common methodologies? for dealing with polymorphism in phylogeny reconstruction and of P1: FIZ/FEA/FGI P2: Fne/FGO QC: FDS/anil T1: FDX September 17, 1999 15:27 Annual Reviews AR093-12 330 WIENS some of the controversies surrounding these methods. I then describe recent studies designed to test the accuracy of these methods and resolve these controversies. I also discuss the impact, although considerably less studied, of within-species vari- ation on comparative studies of the evolution of discrete or qualitative characters. Finally, I review the problem of delimiting species and the operational criteria and methodologies used for delimiting species and distinguishing within and between species variation. PHYLOGENY RECONSTRUCTION General Approaches Polymorphism is important in reconstructing the phylogeny among species for two reasons. First, it is common in data of all types, including morphology, molecules, and behavior. Second, when polymorphism is present, it may have a significant impact on phylogenetic analyses. In particular, various methods for dealing with polymorphism may lead to very different estimates of phylogeny, even when relationships are strongly supported by one or more methods (Figure 1). The abundance and impact of polymorphism are especially clear for closely re- lated species, but different methods for analyzing polymorphic data may affect higher-level relationships as well (e.g. relationships among genera; 138, 139). Yet, surprisingly, the issue of polymorphism is frequently ignored by systematists, particularly those working with morphological and DNA sequence data. Systematists deal with polymorphism, or avoid dealing with polymorphism, in a number of different ways. These general approaches loosely reflect different types of data. Morphologists often exclude characters in which polymorphism is observed, and in fact this is the most common reason given for excluding characters (109a). This practice may be far more common than is apparent from the literature because morphologists rarely provide criteria for excluding or including characters (109a). The next most common exclusion criterion, excluding characters
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